Anya Shortridge walks in her garden in San Diego, Aug. 19. Ms. Shortridge is part of a growing number of gardeners in Southern California who have been delighted to find that planting just a few milkweeds can attract dozens of the majestic black-and-orange monarchs while cutting down on water.

It is one of the worst droughts on record, and the monarch butterflies are lapping it up. Milkweed, that is.

California homeowners are replacing water-greedy grass lawns with the perennial native to California's desert, a happy swap for one species in particular: the monarch, a butterfly that only lays eggs on milkweed.

In recent years, as milkweed has declined, likely due to pesticides use and rapid development, the monarch population has fallen with it. Two decades ago the iconic orange and black insect population was 1 billion strong; Today it hovers below 60 million.

In response to the quick decline, the US Fish and Wildlife Service put forth $1.2 million to bring back the monarchs' habitat, following similar projects nationwide that send milkweed seeds by mail and track breeding grounds in a database. California gardeners are an unexpected but welcome addition to the grassroots effort, and ecologists are hopeful that Californians, who are problem-solving their way through the drought, might help a struggling species in the process.

Nurseries are stockpiling numerous varieties of native milkweed and drawing customers who are adapting to the drought conditions but may also want to bring butterflies to their yards.

At Tom Merriman's native plants nursery in Vista, Calif., business is up 50 percent this season. Until five years ago, Mr. Merriman didn't sell milkweed at all; this summer, he sold more than 14,000 plants and is shipping truckloads of seedlings all over California and other drought-stricken Western states like Arizona, New Mexico, and Utah.

"If you plant it, they will come," Merriman, who has a greenhouse filled with 8,000 milkweed of a dozen varieties, told the Associated Press. "We had chrysalises on shovels, we had them on wheelbarrows. They were up in the nursery on palm trees. They were everywhere, under tables. We were releasing 500 caterpillars a week on native milkweed."

But environmentalists are piping up to say this is not all good news for the monarch.

For one, gardeners may unintentionally opt for the tropical variety, sometimes called "exotic" milkweed, for its colorful pink and yellow flowers.

While the plants may have more curb appeal than the desert types, they bloom all year round which poses a threat to the monarch butterfly's infamous migration patterns.

Native milkweeds go dormant in the winter, and with no place to lay eggs this forces butterflies onward in their migration. The tropical varieties stay in bloom year-round, not giving female butterflies any reason to leave.

North American monarchs travel in the millions every winter from the eastern and central United States and Canada all the way to Mexico, while a smaller number travel through the western US to spend winter along California's Central Coast. The Western monarchs evolved alongside native milkweeds and eating plants from another region might make them more vulnerable, said Hei-ock Kim, of the California Native Plants Society in an interview with the AP.

"When you take plants and animals from where they originated, you're going to change chemistry, climate, biology," she said. "You're changing all their habits, and so things work best when they are where they're supposed to be."

The Minneapolis-based, SaveOurMonarchs, offers free milkweed seeds. In 2015, more than 1 million Milkweed Seed Packets were sent out, according Ward Johnson. He says: "Milkweed Seed Packets will be provided to anyone requesting them."